This invention relates to a mobile radio communication terminal device used in a cellular radio communication system which forms cells as an automobile telephone system, a portable telephone system, or a radio communication LAN system does.
In recent years, cellular radio communication systems have been popularized. In the cellular radio communication system, base stations are distributed over a service area, each base station forms a radio zone called a cell, and a base station and a mobile station are connected by radio in each cell. In this type of system, when the power supply of a mobile station is turned on, the mobile station is first synchronized with the nearest base station and then connected to the latter. Thereafter, the process of switching the base station to which the mobile station is connected, what is called handover, is carried out as the mobile station moves.
For example, in a cellular radio communication system using a CDMA (Code Division Multiple Access) scheme complying with the U.S. TIA standard (IS-95A), when the power supply is turned on at a mobile station, the mobile station tries to pick up the pilot signal broadcast from a base station. It takes about one to two seconds to pick up the pilot signal. This is where the time is required most in the sequence for establishing the system synchronization between the mobile station and base station. After the pilot signal has been picked up, the mobile station establishes the PN code synchronization with the base station and then switches the Walsh code, orthogonal code, thereby receiving a sync channel transmitted from the base station. After receiving the message transmitted through the sync channel, the mobile station demodulates it and stores the received message. The message includes inherent information about the base station and network.
After having completed the synchronization establishing sequence, the mobile station goes into the wait state. In the wait state, the mobile station causes the radio communication circuit section to operate only in the receiving period for a slot where the base station will possibly send a page message to the mobile station and prevents the main clock from being generated in the remaining period to disable the radio communication circuit section. The disabled state is called a sleep state. Such a battery saving operation is called the slot mode operation, whereby the average drawn current of the mobile station is reduced to lengthen the battery service life.
In the wait state, the mobile station not only receives the page message transmitted through the paging channel from the base station to which the mobile station is now being connected, in every receiving period for a slot where the mobile station should receive, but also searches for neighboring base stations in preparation for handover. The base station now being connected reports information about the neighboring base stations to be searched for in the form of a neighbor list message. The neighbor list message is such that the PN offset values of the neighboring base stations are arranged in the order in which they are closer to the base station that transmits the neighbor list message. On the basis of the neighbor list message, the mobile station chooses, for example, three stations in each receiving period for a slot where the mobile station should receive, thereby searching for neighboring base stations.
When neighboring base stations are searched for while the slop mode operation is being carried out, it takes a very long time to complete the search of all the neighboring base stations, because the number of neighboring base stations that can be searched for in a single slot receiving period is as small as about three. Therefore, of the results of searching for all the neighboring base stations, the oldest one is most likely to be unsuitable for the state of the present mobile station. For this reason, if handover were executed with the result of the search being regarded as reliable, the recapture or handover of the pilot signal would fail and the system lost might occur. If the system lost has occurred once, the mobile station starts the operation of establishing the synchronization of the PN code again from scratch as described above. As a result, it takes a very long time for the mobile station to go into the wait state again. In the meantime, the operating current flows in full, thus impairing the battery saving effect in the slot mode operation.
To overcome this problem, when a conventional mobile station has determined that handover is needed, it searches for all the neighboring base stations specified by the neighbor list message again at that time. Then, on the basis of the result of the search, the mobile station chooses the best handover destination and effects handover. It takes as a long time as about several hundred milliseconds to search for all the neighboring base stations again from scratch. Moreover, in the cellular mobile radio communication system, to connect a mobile station to the best base station at all times, the mobile station is generally designed to determine that handover is necessary even when the reception level from the base station now being connected drops slightly. Each time such a determination is made, the mobile station searches for all the neighboring base stations. As a result, the drawn current is still large in the wait state, which makes it difficult to lengthen the continuously using time of the mobile station.
It is, accordingly, a first object of the present invention to provide a mobile radio communication terminal device capable of determining accurately whether a mobile station in the wait state is in a steady state or a moving state.
A second object of the present invention is to provide a mobile radio communication terminal device which enables the best base station to be searched for according to the state of a mobile station and thereby reduces the drawn current in the wait state to lengthen the battery service life.
To achieve the first object of the invention, in a cellular mobile radio communication system where base stations are distributed over a service area, the base stations form their respective radio areas, and a radio channel connects a mobile station and a base station in each of the radio areas, a mobile radio communication terminal device used as the mobile station characterized by comprising: history information storage means for storing information relating to past handovers representing base stations connected to the mobile radio communication terminal device as handover history information; update means for updating the handover history information stored in the history information storage means each time handover is performed; and state determining means for determining whether the mobile radio communication terminal device is in a steady state or a moving state, on the basis of the handover history information stored in the history information storage means.
A concrete configuration of the state determining means is as follows. Each time handover is effected, the state determining means determines whether any of the base stations to which a specific number of past consecutive handovers including the present handover were connected became a handover destination base station in the past, on the basis of the handover history information stored in the history information storage means. When determining that any of the base stations became a handover destination base station, the state determining means recognizes its own mobile radio communication terminal device to be in the steady state. In contrast, when determining that at least one of the base station did not become a handover destination base station, the state determining means recognizes its own mobile radio communication terminal device to be in the moving state.
Therefore, with the present invention, since the present state of its own station is determined by tracing the history of past handovers, it is possible to determine accurately with relative ease whether its own station is now in the moving state or is now stationary or in a limited range even when moving, or in what is called a steady state.
To accomplish the second object of the present invention, the mobile radio communication terminal device is characterized by, in addition to the history information storage means and state determining means, further comprising base station searching means for selecting a different algorithm, depending on either the steady state or the moving state, according to the result of the determination by the state determining means and searches for a base station.
The configuration of the base station searching means is as follows.
In the steady state, the base station searching means classifies a first number of base stations in the handover destination base stations stored in the history information storage means into a first search group and a second number of the other base stations larger than the first number into a second search group and searches the first search group more frequently than the second search group.
In the moving state, the base station searching means selects a third number of neighboring base stations closer in distance to the base station now being connected on the basis of a list of neighboring base stations reported from the base station to which its own station is now being connected, classifies not only these neighboring base stations into a third search group but also the other neighboring base stations in the list into a fourth search group, and searches the third search group more frequently than the fourth search group.
Therefore, with the present invention, use of a search algorithm suitable for the characteristic in each of the steady state and moving state enables a base station to be searched for efficiently.
For example, because in the steady state, the mobile station is either stationary or moving slowly in a limited range, the number of base stations that will possibly become handover destinations is about three to four at most. Therefore, in the steady state, just searching for a limited number of base stations during handover enables a suitable base station to be found as the handover destination. Consequently, as compared with the case where a large number of neighboring base stations are searched for unconditionally, a suitable handover destination can be found in a shorter search time. This shortens the search time, reducing the power consumption that much, which lengthens the battery service life.
In contrast, in the moving state, the base station most suitable for the handover destination changes dynamically. Because of this, in the moving state, neighboring base stations are searched for over a relatively wide range during handover according to the neighbor list message reported from the base station now being connected, thereby making it possible to find a suitable base station as the handover destination with a high probability.
Furthermore, the base station searching means is characterized by including means for determining whether handover is needed and, when the determining means determines that handover is not needed, searching for each base station belonging to the first search group and each base station belonging to the second search group alternately.
In addition, the base station searching means is characterized by, when the determining means determines that handover is not needed, searching for the third number of neighboring base stations belonging to the third search group and the fourth number of neighboring base stations smaller than the third number selected from the fourth search group alternately.
With this configuration, in each reception slot, not only each base stations in the first or third search group but also each base stations in the second or fourth search group can be searched for. This prevents the result of searching for the base stations in the second or fourth search group from becoming too old. Thus, when handover is needed, a suitable handover destination can be found with a relatively high probability.
Furthermore, the base station searching means is characterized by including necessity determining means for determining whether handover is needed and, when the necessity determining means determines that handover is needed, searching for all the base stations belonging to the first or third search group and by, when a suitable base station could not be found as the result of the search, searching for each base station belonging to the second and fourth search group.
With this configuration, when a suitable base station has been found as the handover destination as the result of searching for base station belonging to the first or third search group, the second or fourth search group need not be searched. Therefore, as compared with the case where all the search groups are searched unconditionally, the average search time is shortened, making shorter the time required for the handover process. This reduces the power consumption that much, which lengthens the battery service life.
Still furthermore, the base station searching means is characterized by, when a suitable base station could not be found as the handover destination as the result of the search of the first or third search group, causing the necessity determining means to determine again whether handover is needed and by, only when the determination has shown that handover is needed, searching the second or fourth search group.
With this configuration, when it is determined whether the quality of the reception from the base station now being connected is restored, after the search of the first or third search group, and handover is no longer needed because of the restoration of the reception quality, the mobile station can return to the sleep state without searching the second or fourth search group. Therefore, as compared with the case where the search of all the search groups is continued unconditionally, a useless search operation is eliminated, thus reducing the power consumption that much, which lengthens the battery service life.
Still furthermore, the base station searching means is characterized by, when a suitable base station could not be found as the handover destination as the result of searching the first or third search group and the second or fourth search group, searching the first or third search group and the second or fourth search group repeatedly and by, when the number of the repetitions has reached a predetermined number of times, performing variable control of the determination conditions of the means for determining whether handover is needed.
With this configuration, when a possibility of handover is checked excessively because the condition for determining whether handover is needed has not been optimized, the handover determination condition can be corrected automatically to the optimum condition. As a result, a base station is searched for only when handover is really needed, reducing the power consumption, which lengthens the battery service life.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantage s of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.